1. Field of the Invention
The present invention relates generally to an automatic automotive transmission and more specifically to an improved cruise control arrangement therefor which obivates frequent downshift/upshift hunting upon the vehicle ascending a slope or the like.
2. Description of the Prior Art
FIG. 1 of the drawings shows previously proposed transmission control arrangement (disclosed on pages 80-81 of Datsun Blue Bird Service Journal No. 474 October 1982 published by Nissan Motor Co.). In this arrangement a transmission control circuit 1 (e.g. a hydraulic spool valve control circuit) receives inputs from a vehicle speed sensor 2 and a throttle position sensor 3 which outputs a signal indicative of the opening degree of the throttle valve 4 of the engine associated with the transmission 5. In this system the transmission 5 is comprised of planetary gear sets, hydraulic clutches and brakes and capable of producing four forward speeds. The system further includes an automatic vehicle speed control device (ASCD) 6 which also receives input from the vehicle speed sensor 2, and a throttle valve actuator 7 which controls the position of the throttle valve 4 in place of the normal manual control, once the system has entered so the called "cruise control" mode of operation.
However, this system has tended to suffer from a drawback in that the transmission control circuit and the automatic vehicle speed control device (ASCD) are independently arranged and accordingly function independently of each other, whereby when the vehicle in which the above described transmission system is mounted is operated under "cruise control" and ascends a slope the inclination of which is sufficient to reduce the vehicle speed, the vehicle speed control device opens the throttle of the engine to the point whereat the transmission control circuit, in accordance with its own control schedule, induces a 4-3 downshift irrespective of the ASDC issuing a command to this effect. Viz., as shown in FIG. 2, if the speed falls below the vehicle speed selected to be maintained by the cruise control, a relatively small increase in the opening of the throttle valve reaches the threshold at which the transmission control circuit will, according to its independent control, induce a 4-3 downshift. Subsequently, as the vehicle speed increases due to the downshift, the vehicle speed soon returns to the preselected one. However, to prevent vehicle speeds in excess of the desired value, the automatic vehicle speed control device begins closing the throttle valve. Accordingly, the threshold at which a 3-4 upshift is induced by the transmission control circuit is soon reached and the transmission is upshifted.
If the increased load which originally induces the vehicle speed reduction is still present, viz., the vehicle is still ascending the slope, the vehicle speed begins to reduce again. This induces hunting between third and fourth speeds to occur at approximately 3-4 second intervals as shown in FIG. 3.
This, while maintaining the desired vehicle speed tends to be highly disconcerting to the vehicle passengers.
To overcome this hunting problem it has been arranged for (as shown in FIG. 1) the transmission to issue a signal to the automatic vehicle speed control device when the transmission control circuit induces a 4--3 downshift. This signal triggers a timer in the ASCD and causes the latter to energize a solenoid in the transmission which inhibits a 3-4 upshift for a preselected period of 10 to 20 seconds (for example).
This while overcoming the hunting problem incurs the drawback that the control schedule according to which the transmission is controlled is extremely complex due to the essentially independent natures of the transmission control circuit and the vehicle speed control device and the interaction which inherently occurs therebetween.